1. Field of the Invention
The present invention relates to a system for monitoring a workpiece during laser shock processing, and in particular, a system for monitoring the pressure pulse applied to a workpiece during laser shock processing.
2. Description of the Related Art
Laser shock processing involves a pulse of coherent radiation to a piece of solid material (workpiece) to produce shockwaves therein. The produced shockwave cold works the solid material to impart compressive residual stresses within the solid material. These compressive residual stresses improve the fatigue properties of the solid material.
Current laser shock processing utilizes two overlays: a transparent overlay (usually water), and an opaque layer (usually an oil based paint or black plastic tape). During processing, a laser beam is directed to pass through the transparent overlay and is absorbed by the opaque layer, causing a rapid vaporization of the opaque layer (plasma creation) and generation of a high-amplitude shockwave. The shockwave cold works the surface of the part and creates deep compressive residual stresses which provide an increase in fatigue properties of the workpiece. A workpiece is typically processed by employing a matrix of overlapping spots that cover the fatigue-critical zone of the part.
Currently, there is no known real-time method for measuring the shock pressure applied to a workpiece during laser shock peening. While commercial pressure gauges, such as special quartz gauges or PVDF gauges are available to make pressure measurements, these gauges must be used offline (not in real time on a workpiece). Furthermore, these gauges are single-use devices.
A quartz gauge is based on the piezoelectric behavior of quartz crystals. When a pressure is applied to one surface of a quartz crystal, an electric current proportional to the stress difference between this surface and the opposite surface is produced between electrodes attached to these surfaces. The current then passes through a resistor and the voltage measured across the resistor is proportional to the difference in the stress between the opposite surfaces. In the "thick gauge" mode, most or all of the shockwave passes into the thickness of the gauge before it reaches the opposite surface of the crystal. This enables one to measure the entire shockwave profile directly.
One problem with current laser shock processing systems is that there is no real-time method or apparatus for measuring shock pressure or plasma characteristics during laser shock peening or correlating them to the imparted deep compressive residual stresses in a workpiece. Previous methods of measuring a pressure pulse applied to a workpiece included use of a quartz gauge. The disadvantage of using a quartz gauge is that a quartz gauge is a single use instrument. In addition, the use of a quartz gauge does not permit real-time measuring of a pressure pulse while processing a workpiece. The use of a quartz gauge is limited to measuring the pressure pulse applied to a workpiece either before or after laser shock processing (i.e., not real time).
Another problem in the art is that there is no method for correlating plasma characteristics to a pressure pulse applied to a workpiece.
Another problem in the art is that there is no known method or apparatus for real-time determination of imparted compressive residual stresses in a workpiece during laser shock peening. Currently, the method of evaluating an imparted residual stress profile is to measure the residual stresses using x-ray diffraction techniques. In order to use x-ray diffraction, a workpiece is normally removed from the laser shock processing station and placed in an x-ray machine, wherein an x-ray beam is directed to the workpiece surface to measure the residual stresses at that surface. In order to get an in-depth profile, a sequence of thin layers is removed from the surface by electropolishing, then the surface residual stresses are measured between each electropolishing step. If only the residual stress at the original surface of the workpiece is measured, the measurements also includes the unknown effects of previous surface finishing processes. These usually vary from part to part and could be differentiated from the stresses imparted by laser shock peening. The technique of x-ray diffraction for in-depth profiles is a destructive method for evaluating compressive residual stresses imparted in a workpiece by laser shock peening. The use of x-ray diffraction is limited to post-laser shock peening analysis. Therefore, x-ray diffraction cannot be used as a real time method for determining imparted compressive residual stresses.